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Superfluid helium nanodroplets are often considered as transparent and chemically inert nanometer-sized cryo-matrices for high-resolution or time-resolved spectroscopy of embedded molecules and clusters. On the other hand, when the helium nanodroplets are resonantly excited with XUV radiation, a multitude of ultrafast processes are initiated, such as relaxation into metastable states, formation of nanoscopic bubbles or excimers, and autoionization channels generating low-energy free electrons. Here, we discuss the full spectrum of ultrafast relaxation processes observed when helium nanodroplets are electronically excited. In particular, we perform an in-depth study of the relaxation dynamics occurring in the lowest 1s2s and 1s2p droplet bands using high resolution, time-resolved photoelectron spectroscopy. The simplified excitation scheme and improved resolution allow us to identify the relaxation into metastable triplet and excimer states even when exciting below the droplets' autoionization threshold, unobserved in previous studies.

The relative phase of first ( ω 1 ) and third harmonics ( ω 3 ) extreme ultraviolet light pulses was varied to control the population of the 2 s 2 state in helium through the interference of ω 1 + ω 1 and ω 3 − ω 1 two-photon excitation paths. The population was monitored by observing the total electron yield due to the 2 s 2 autoionization decay. Maximum yield occurs when the relative phase of the two harmonics matches the phase difference of complex atomic amplitudes governing the two excitation paths. The calculated trend of atomic phase differences agrees well with the measured data in the spectral region of the resonance, provided that time-reversed − ω 1 + ω 3 path is also taken into account. These results open the way to accessing phase differences of two-photon ionization paths involving energetically distant intermediate states and to perform interferometry in the extreme ultraviolet range by monitoring final state populations.

An open-loop control scheme of molecular fragmentation based on transient molecular alignment combined with single-photon ionization induced by a short-wavelength free electron laser (FEL) is demonstrated for the acetylene cation. Photoelectron spectra are recorded, complementing the ion yield measurements, to demonstrate that such control is the consequence of changes in the electronic response with molecular orientation relative to the ionizing field. We show that stable C2H2+ cations are mainly produced when the molecules are parallel or nearly parallel to the FEL polarization, while the hydrogen fragmentation channel (C2H2+ → C2H+ + H) predominates when the molecule is perpendicular to that direction, thus allowing one to distinguish between the two photochemical processes. The experimental findings are supported by state-of-the art theoretical calculations.

The super-bandgap laser irradiation of the in situ prepared As-S chalcogenide films was found to cause drastic structural transformations and unexpected selective diffusion processes, leading to As enrichment on the nanolayer surface. Excitation energy dependent synchrotron radiation photoelectron spectroscopy showed complete reversibility of the molecular transformations and selective laser-driven mass transport during "laser irradiation"-"thermal annealing" cycles. Molecular modeling and density functional theory calculations performed on As-rich cage-like clusters built from basic structural units indicate that the underlying microscopic mechanism of laser induced transformations is connected with the realgar-pararealgar transition in the As-S structure. The detected changes in surface composition as well as the related local and molecular structural transformations are analyzed and a model is proposed and discussed in detail. It is suggested that the formation of a concentration gradient is a result of bond cleavage and molecular reorientation during transformations and anisotropic molecular diffusion.

The hitherto unexplored two-photon doubly excited states [Ne^{*}(2p^{-1}3s)]_{2} were experimentally identified using the seeded, fully coherent, intense extreme ultraviolet free-electron laser FERMI. These states undergo ultrafast interatomic Coulombic decay (ICD), which predominantly produces singly ionized dimers. In order to obtain the rate of ICD, the resulting yield of Ne_{2}^{+} ions was recorded as a function of delay between the extreme ultraviolet pump and UV probe laser pulses. The extracted lifetimes of the long-lived doubly excited states, 390(-130/+450) fs, and of the short-lived ones, less than 150 fs, are in good agreement with ab initio quantum mechanical calculations.

The dipole forbidden inner-shell excitation of the N 2 molecule to the (1s) −1 (2pπ) 3 Π state and its decay have been studied in an electron impact experiment at variable incident energy. The contribution of the dipole forbidden transitions to the N 2 autoionization spectrum has been isolated via two different techniques. One is based on a subtraction procedure relying on non-coincidence measurements, while the other consists of an experiment where the electrons ejected in the decay process are detected in coincidence with scattered electrons which suffered an energy loss equal to the excitation energy of the (1s) −1 (2pπ) 3 Π state.

Nanostructured titanium dioxide (ns-TiO2) films were grown by supersonic cluster beam deposition method. Transmission electron microscopy demonstrated that films are mainly composed by TiO2 nanocrystals embedded in an amorphous TiO2 phase while their electronic structure was studied by photoemission spectroscopy. The cluster assembled ns-TiO2 films are expected to exhibit several structural and chemical defects owing to the large surface to volume ratio of the deposited clusters. Ultraviolet photoemission spectra (hv = 50 eV) from the valence band unveil the presence of a restrained amount of surface Ti 3d defect states in the band gap, whereas Ti 2p core level X-ray photoelectron (hv = 630 eV) spectra do not manifestly disclose these defects.

Electron impact has been used to excite dipole forbidden inner-shell transitions of CO which are not accessible by photon impact. Coincidence techniques have then been used to study the decay of the C 1s-2pi (triplet PI) inner-shell excited state, and to isolate this autoionization spectrum from that of the (singlet PI) inner-shell excited state. An ab initio multiconfiguration and multicentre calculation allowed the assignment of the spectral features to the various participator and spectator transitions leading to the final CO+ doublet and quartet states. Although the same doublet states are populated from both the singlet and triplet intermediate states, the experimental intensities and energies are significantly different due to Franck–Condon effects.

It is an established practice to combine experimental spectroscopic results and quantum chemical data in order to achieve information on the internal motions and structure of isolated molecules. Quantum mechanical calculations support the assignment of experimental data providing trial spectroscopic parameters that guide the initial research. At the same time, experimental results can be used as benchmarks to validate and test the accuracy of the theoretical methods. Quantum chemical calculations also allow for an overall depiction of the potential energy surfaces that drive the intra-molecular motions. Adjusting such estimated surfaces on the basis of the observed findings allows the characterization the molecular behaviour. We used this approach to study the conformational and tautomeric equilibria of several isolated molecules, from their microwave and X-ray spectra: (i) the interpretation of "composite" PES and NEXAFS data was attempted on the basis of quantum chemical calculations, (ii) rotational characterization of the molecular structures was combined with the quantitative information achieved by XPS to determine the identity and the abundances present; in the case of XPS where the sample is at thermal equilibrium the latter piece of information can be used to estimate the energy differences among the species in the sample.

We present a combined experimental and theoretical investigation of the electronic spectra of barbituric acid (BA) and 2-thiobarbituric acid (TBA) molecules in the vapour phase. The valence band photoemission and the core level (O, N, C 1s and S 2p) photoemission and photoabsorption spectra have been measured in the vapour phase using synchrotron light and then assigned with the support of quantum chemical calculations. First the valence band binding energies have been calculated using the Koopman's approximation at the B3LYP/6- 311++G** level, then the outer valence behaviour has been better approximated with electron propagator calculations (OVGF and P3 approximations), available in the G09 software package. [1] The core ionized states and core-to-excited states transitions were solved within the Hartree-Fock approximation by explicitly taking into account the core hole, using the GSCF3 code developed by prof. N. Kosugi. [2] The calculations were run for both the tri-keto and the hydroxy-di-keto form, where the enolization of one of the two equivalent keto groups is produced by proton transfer from the methylene group, which is more acidic than the NH groups. In agreement with previous ab initio [3, 4] and gas-phase electron diffraction [5] works, only the tri-keto tautomers have been detected. [1] Gaussian 09, Revision A.02, M. J. Frisch et al., Gaussian, Inc., Wallingford CT, 2009. [2] N. Kosugi, and H. Kuroda, Chem. Phys. Lett. 74, 1980, 490. N. Kosugi, and H. Kuroda, Chem. Phys. Lett., 94, 1983, 377. N. Kosugi, Theor. Chim. Acta, 72, 1987, 149. [3] F. Zuccarello, G. Buemi, C. Gandolfo and A. Contino, Spectrochim Acta A, 59, 2003, 139-151. [4] E. Méndez, M. F. Cerdá, J. S. Gancheff, J. Torres, C. Kremer, J. Castiglioni, M. Kieninger, and O. N. Ventura, J. Phys. Chem. C, 111, 2007, 3369-3383 [5] O. V. Dorofeeva, I. I. Marochkin, N. M. Karasev, I. F. Shishkov and H. Oberhammer, Structural Chemistry, 22, 2011, 419-425

The adsorption of CO and the reaction of CO with preadsorbed oxygen at room temperature has been studied on the Rh(1 1 1)2 × 1-O surface and on ceria–Rh(1 1 1) “inverse model catalyst” surfaces using C 1s and O 1s core level and valence band photoelectron spectroscopy with synchrotron radiation. The adsorption of CO on the oxygen-precovered (2 × 1)-O surface was found to proceed slower than on the clean Rh(1 1 1) surface, because of a kinetic limitation which is introduced by the O + CO clean-off reaction. The latter removes adsorbed oxygen as CO 2 at 300–320 K, which desorbs into the gas phase. On the ceria–Rh(1 1 1) surfaces the O + CO oxidation reaction is much faster than on the Rh(1 1 1)2 × 1-O surface suggesting a catalytically active role of the ceria–Rh interface. The XPS spectra indicate a preferential occupation of hollow-type CO adsorption sites on the ceria–Rh(1 1 1) surface, which might be located at the oxide–metal interface. No CO dissociation has been detected on the Rh(1 1 1) supported ceria inverse catalyst surfaces. On a CeRh 3 alloy surface, prepared by thermal decomposition of the ceria in ultrahigh vacuum, the hollow-type CO adsorption sites are energetically favoured suppressing the on-top sites almost completely, but the global adsorption energy is lower than on Rh(1 1 1) as indicated by the reduced CO saturation coverage.

Vibrationally resolved N2+ (B–X) (1–2) and (0–1) emission bands were recorded after photoexcitation of N2 in the 20–46 eV energy range. The branching ratio between these transitions reveals strong non-Franck–Condon effects. Ab initio calculations using a configuration interaction method based on Kohn–Sham orbitals indicate that the autoionisation of certain superexcited states is responsible for some structures present in the branching ratio curve.

The near edge x-ray absorption spectra of CX4 (X = H, F, Cl) have been measured with high resolution at the 1s edges of carbon and fluorine, and the 2p edge of chlorine. The absolute photoabsorption cross section has been determined at the resonances and in the nearby continuum. In accord with previous work, the antibonding valence peaks of the halides do not show any vibrational structure but sharp Rydberg states are observed at the C 1s edge. We report the first detailed study of the CCl4 Rydberg states. The vibrational frequency of the CF4 Rydberg states is anomalously high, which we attribute to a Fermi resonance. The C 1s line widths were compared to theoretical predictions of ion state line widths and reasonable values are found for CH4 and CCl4, but for CF4 the measured value is higher than the theoretical value. The discrepancy is assigned to mixing of valence band character in the Rydberg states, which is especially strong in CF4, and to multi-centre autoionization. Finally the integrated oscillator strength below threshold is shown to vary in the order CF4 > CH4 > CCl4.

Fluorescence decay of the 2s2p(6)np (1)P(o) excited states of neon has been observed directly for the first time. This process has also been observed indirectly by threshold photoelectron spectroscopy and in electron/photon coincidences. The measurements have allowed this weak decay channel to be characterized and branching ratios derived. The fluorescence excitation spectrum is well described by a simple Rydberg model that highlights the interplay and competition between the decay channels of autoionization and fluorescence.

(1999). Core-hole line widths and the resolution of soft x-ray monochromators. Synchrotron Radiation News: Vol. 12, No. 4, pp. 27-30.

A near-edge X-ray absorption fine-structure (NEXAFS) study of the O K→Π ∗ excitation in CO (534 eV) and O 2 (531 eV) molecules has been carried out. A Franck–Condon analysis of the transitions has been accomplished assuming a Morse potential energy curve for the core hole states. The vibrational constants of O 2 are in agreement with values expected from the equivalent core molecule OF. The values of the lifetime widths of the Π ∗ excited states are Γ O 2 =149±10 meV and Γ CO =158±8 meV. The measured lifetime width of O 2 is an upper limit to the true value as unresolved spin–orbit structure is believed to be present, which may cause an additional broadening of ∼5 meV. In spite of this, it is substantially smaller than the hitherto accepted value of 180 meV, necessitating a re-evaluation of most previously published data.

Angle-resolved ${M}_{4,5}{N}_{4,5}{N}_{4,5}$ Auger spectra of xenon have been measured near the Xe $3d$ threshold by using synchrotron radiation excitation. The angular anisotropy parameters extracted from the experimental data generally agree with the recent theoretical results based on multiconfiguration Dirac-Fock calculations. The angle-resolved measurements have also been employed in order to study the character of the resonance structures below the ${M}_{4}$ threshold.

This paper reports the present stage of commissioning of the gas-phase photoemission beamline at Elettra, Trieste. The beamline is designed for atomic and molecular science experiments with high-resolution and high-flux synchrotron radiation. It consists of an undulator source, variable-angle spherical-grating monochromator and two experimental stations. The design value of the energy range is 20 to 800 eV with a specified resolving power of over 10000. The procedure adopted for calibration of this type of monochromator is discussed. At present a resolving power up to 20000 and a range up to 900 eV have been measured. Absorption spectra taken at the argon L II,III-edge and at the nitrogen, oxygen and neon K-edges are as sharp as, or sharper than, any reported in the literature. The instrumental broadening is well below the natural line-width making it difficult to quantify the resolution; this problem is discussed.

Three different teams have recently measured Si2 p high resolution synchrotron radiation spectra from the ‘prototypical’7 × 7 reconstruction of the clean Si(111) surface. Although nearly identical line shapes were obtained, the model functions used in the analysis were rather different, leading to somewhat dubious attributions of each surface shifted component to a specific stuctural building block of the7 × 7 reconstruction. To objectively test these model functions we compare the curve synthesis resulting from a new set of high resolution spectra acquired independently. This comparison favors the model with five surface shifted components and consequently supports the corresponding assignments.

When exposed to ultraintense x-radiation sources such as free electron lasers (FELs) the innermost electronic shell can efficiently be emptied, creating a transient hollow atom or molecule. Understanding the femtosecond dynamics of such systems is fundamental to achieving atomic resolution in flash diffraction imaging of noncrystallized complex biological samples. We demonstrate the capacity of a correlation method called "partial covariance mapping'' to probe the electron dynamics of neon atoms exposed to intense 8 fs pulses of 1062 eV photons. A complete picture of ionization processes competing in hollow atom formation and decay is visualized with unprecedented ease and the map reveals hitherto unobserved nonlinear sequences of photoionization and Auger events. The technique is particularly well suited to the high counting rate inherent in FEL experiments.

The photoabsorption spectra of neon and argon in the energy region of the and doubly excited states have been studied. The resolution and flux of the new VUV undulator beam line at ELETTRA allowed the resolution of new features for Ne and the first observation for Ar. A comparison of the measured photoabsorption spectra and of the threshold photoelectron spectra in the energy region of the and satellite states provides information on the decay routes of the inner-shell doubly excited states.

High resolution resonant Auger spectra have been measured at the vacuum-ultraviolet beam line at Sincrotrone Trieste while tuning the photon energy across the 2{ital p}{sub 3/2}{sup {minus}1}3{ital d} resonance of argon. A large variation of the branching ratio for decay into the spectator 3{ital p}{sup {minus}2}3{ital d} and shakeup 3{ital p}{sup {minus}2}4{ital d} channels is observed. The effect has not been seen previously in resonant inner-shell excitation and is attributed to the interference between the direct photoionization process and resonant Auger decay leading to the same final state. {copyright} {ital 1996 The American Physical Society.}

Atomic force microscope(AFM) induced local oxidation is a versatile and promising nanofabrication process used successfully to produce quantum devices. Nevertheless, little information is available on the chemical and structural properties of the grown oxide. We address this open issue by a spectromicroscopic study of nanoscopic oxide patterns grown by AFM anodic oxidation on n-type silicon substrate. We show that AFMoxidation produces chemically uniform, stoichiometric SiO2, and that its chemical and structural properties do not depend on the applied voltage. The observed electrostatic shift of the oxide binding energies allows a simple estimation of the electrical properties of the AFM induced oxide.

Photoelectron spectroscopy with synchrotron radiation, low energy electron diffraction, and ion-scattering spectroscopy were used in order to study the Ti/MgCl(2) interface grown on an atomically clean Si(111) 7 × 7 substrate. A series of high resolution spectra after deposition of a thick MgCl(2) layer, step by step deposition of Ti and gradual annealing, indicated a very reactive interface even at room temperature. Strong interaction between the incoming Ti atoms and the MgCl(2) layer, leads to the formation of Ti(2+) and Ti(4+) oxidation states. The interfacial interaction continues even at multilayer Ti coverage mainly by the partial disruption of Mg-Cl bonds and the formation of Ti-Cl sites, rendering this interface a very promising UHV-compatible model of a pre-catalyst for olefin polymerization. After the final annealing, the MgCl(2) multilayers desorb while Ti remains on the surface forming a silicide layer on which Cl and Mg atoms are attached.

Laser orientation in the initial state has been used to study the properties of satellite transitions in inner-shell photoionization of rubidium atoms. The linear magnetic dichroism in the angular distribution (LMDAD) has been utilized to probe the continuum waves of orbital angular momentum conserving monopole, and angular momentum changing conjugate satellites, accompanying the $4p$ ionization of atomic Rb. We show experimentally that LMDAD of both types of satellite transitions is nonzero and that LMDAD of monopole satellites, measured as a function of photon energy, mimics the LMDAD of direct photoionization, whereas the LMDAD of conjugate transitions deviates drastically from that trend. The results indicate that conjugate transitions cannot be described theoretically without explicit inclusion of electron-electron interaction. The present data can thus be used as a very precise test of current models for photoionization.

Photoelectron spectroscopy with synchrotron radiation (SRPES), temperature programmed desorption (TPD), low energy electron diffraction (LEED), and ion-scattering spectroscopy (ISS) were used in order to study the MgCl(2)/Ti(0001) interface. A clear hexagonal LEED pattern confirmed the presence of a quite large grain of Ti(0001) on the substrate while no new superstructure was formed after deposition of MgCl(2) either at room or at elevated temperatures. A series of high resolution spectra after step by step MgCl(2) deposition and gradual annealing indicated strong interaction between MgCl(2) and the substrate while ISS measurements showed that there is no migration of Ti atoms into the deposit layers. Additional quantities of deposited MgCl(2) grew stoichimetrically on top of the chemically active interface. Annealing at approximately 350 degrees C caused clustering of the MgCl(2) multilayer and TPD results showed that they desorbed stoichimetrically at temperatures between 360 and 380 degrees C. The interfacial TiCl(x)Mg(y) species dissociated by the disruption of the Cl-Mg bonds at temperatures higher than 400 degrees C and metallic Mg evaporated. The Cl atoms remained attached on the Ti surface but they did not form any ordered structure even after annealing at 730 degrees C. The present results indicate the occurrence of charge transfer at the Ti/MgCl(2) interface through the Cl ligands and provide valuable information for catalyst design.

We present spectra of triplet and singlet metastable helium atoms resonantly photoexcited to doubly excited states. The first members of three dipole-allowed 1,3Po series have been observed and their relative photoionization cross sections determined, both in the triplet (from 1s2s 3Se) and singlet (from 1s2s 1Se) manifolds. The intensity ratios are drastically different with respect to transitions from the ground state. When radiation damping is included the results for the singlets are in agreement with theory, while for triplets spin-orbit interaction must also be taken into account.

We have studied the adsorption of low dimensional gold on ceria, produced by evaporation onto the surface. The interaction of gold with CeO(2)(111) layers was investigated with x-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, and resonant photoelectron spectroscopy (RPES). Gold was deposited in steps onto a 1.5 nm thick CeO(2)(111) layer epitaxially grown on a Cu(111) substrate. The RPES showed a partial Ce(4+)--Ce(3+) reduction, observed as a resonant enhancement of the 4f level of the Ce(3+) species. This can be explained by possible creation of a new Au(+) ionic state. The observed effects are stronger for Au deposition at room temperature than at 250 degrees C. The obtained results are in agreement with already published density functional theory calculations reporting weakening of bond between the oxygen and the Ce atoms in ceria caused by the presence of gold.

We present spectra of triplet and singlet metastable helium atoms resonantly photoexcited to doubly excited states. The first members of three dipole-allowed ;{1,3}P;{o} series have been observed and their relative photoionization cross sections determined, both in the triplet (from 1s2s ;{3}S;{e}) and singlet (from 1s2s ;{1}S;{e}) manifolds. The intensity ratios are drastically different with respect to transitions from the ground state. When radiation damping is included the results for the singlets are in agreement with theory, while for triplets spin-orbit interaction must also be taken into account.

The energy dependence of the fluorescence intensity from the $4{p}^{4}(^{1}D)5p$ states of Kr was studied experimentally across the $3{d}_{5/2}^{\ensuremath{-}1}5p$, $3{d}_{3/2}^{\ensuremath{-}1}5p$, and $3{d}_{5/2}^{\ensuremath{-}1}6p$ resonances. The results of multiconfiguration Dirac-Fock calculations were compared with the measurements and with earlier calculations. The calculated interference effects in the resonant Auger process were found to be very sensitive to the initial state configuration interaction. The addition of the $7p$ orbital in the wave functions did not improve the agreement between experiment and calculation. The effects of cascade processes were also considered with the help of Hartree-Fock calculations.

Visible-UV fluorescence emission of gas-phase hydrogen sulfide, H(2)S, has been studied at the S 2p edge with synchrotron radiation excitation. Dispersed fluorescence measurements in the wavelength range 300-900 nm were taken at several photon energies corresponding to the excitations of the S 2p electrons to the unoccupied molecular and Rydberg orbitals. The spectra reveal fluorescence from the H, S, S(+), HS and HS(+) photo-fragments. H is found to be the strongest emitter at Rydberg excitations, while the emission from S(+) is dominant at the molecular resonances and above the S 2p ionization thresholds. The intensities of hydrogen Lyman-alpha (122 nm), Balmer-alpha (656 nm), Balmer-beta (486 nm) transitions as well as the visible-UV total fluorescence yield (300-900 nm) and the total ion yield were measured by scanning the photon energy in small steps across the S 2p edge. The different Balmer lines show some sensitivity to the specific core excitations, which is, however, not so strong as that recently observed in the water molecule [E. Melero Garcia, A. Kivimaki, L. G. M. Pettersson, J. Alvarez Ruiz, M. Coreno, M. de Simone, R. Richter and K. C. Prince, Phys. Rev. Lett., 2006, 96, 063003].